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Young Canadian Eugenie Bouchard didn't care why she was dominating a Grand Slam winner and top-10 player, only that she had finally broken through for the biggest win of her career.

Bouchard, 19, defeated her highest-ranked opponent ever when former U.S. Open champ Samantha Stosur withdrew midmatch with apparent right leg problems.

Bouchard, the reigning Wimbledon Junior singles champion, took a big lead on the hurting Stosur, who retired from the match trailing 6-1, 2-0.

Bouchard noticed the wrap on Stosur's leg and that her accomplished opponent wasn't chasing down several balls she normally would reach easily. Still, Bouchard felt she'd played a brand of tennis up to any challenge.

"I believe that I can beat these players," Bouchard said. "It's time to happen."

The ninth-ranked Stosur was out of sorts against Bouchard from the start. The Australian had her right leg wrapped and spoke with a trainer between sets before going back out. But two games in, the former U.S. Open champ called her opponent over, wished her well and ended the match.

An earlier rain delay of nearly 7 1/2 hours washed out matches of the tournament's top draws: Serena Williams and her sister Venus.

Serena, riding an 11-match winning streak here, was scheduled to play American qualifier Mallory Burdette. Venus will take on Varvara Lepchenko for a spot in the quarterfinals. Second-seeded Caroline Wozniacki moved on without hitting a shot when opponent Andrea Petkovic withdrew with a calf injury a few hours before their scheduled match.

The rain fell steadily at the Family Circle Tennis Center until dinnertime, when crews were finally able to start drying the large puddles on the gloppy, green clay surfaces.

Once play began, ninth-seeded Jelena Jankovic - the 2007 champion here - needed just 64 minutes to oust 19-year-old American Jessica Pegula 6-0, 6-4. Pegula, whose father Terry owns the Buffalo Sabres, won the first two professional matches of her career this week before falling to the former world No. 1.

Stefanie Voegele defeated 10th-seeded Julia Goerges, 3-6, 6-1, 6-3.

The delay means the year's first clay-court event has some ground to make up to finish by Sunday. Without more rain, there would still be four round-of-16 matches to be played on Friday before the quarterfinals were set.

Serena Williams, who was not sharp despite winning her opening match over Camila Giorgi on Tuesday, will have waited almost three days to play again. Venus defeated 19-year-old Monica Puig in three sets Wednesday night. The sisters would both double up Friday if they won early matches.

Win a second time and it would mean a Williams' sister semifinal Saturday. They have not met in a tournament since 2009.

Both have enjoyed championship moments at the Family Circle. Serena won titles in 2008 and 2012. Venus won the crown in 2004 and last September clinched the winning points as the Washington Kastles took the World Team Tennis title on Billie Jean King Court.

Others who had matches delayed were sixth-seeded Lucie Safarova against 11th-seeded Sorana Cirstea and the all-American match of Madison Keys and Betthanie Mattek-Sands.

Cirstea, like Serena Williams, hasn't played since winning on Tuesday.

It took tournament crews more than three hours to dry the stadium court enough for Stosur and Bouchard to close the chilly, damp night. Tournament director Bob Moran said the clay surface sopped up plenty of water during the steady rain. "Our court maintenance team worked extremely hard to get the court ready and suitable for play," he said.

Jankovic said it was difficult keeping sharp and loose as the delay stretched through the afternoon. "I wanted to take a nap, but I couldn't," she said. "We joked that if they rained it out, we have a go-shopping day."

Instead, Jankovic felt fortunate to move to the quarterfinals when so many other competitors will likely have to play twice Friday. "At the end of the day, I was lucky," she said. "I came out and played quite well."

Jankovic will face Bouchard on Friday for a spot in the semifinals. Wozniacki takes on Voegele, the Dane trying for her eighth consecutive match win here.

Petkovic was Jankovic's doubles partner this week. Jankovic had enjoyed the pairing so far and looked forward to seeing what they could to here. Instead, Petkovic said in a statement she might do more damage to her leg if she competed.

"It is a very beautiful tournament and I loved every part of it here," Petkovic said. "So I hope to come back under better conditions."